The durability of adhesives for wood structural purposes is closely regulated by, for instance, the American National Standards Institute (ANSI); the American Society for Testing Materials (ASTM) and the Canadian Standards Association (CSA). Polyurethane adhesives must meet these standards if they are to find utility as adhesives in structural wood products.
C. B. Vick and E. A. Okkonen in Strength and Durability of One-Part Polyurethane Adhesive Bonds to Wood, Forest Products Journal, 48 (11-12), 1999, pages 71-76, showed that, whilst the dry adhesion strength of polyurethane adhesives to wood is comparable to other adhesive chemistries, the level of wood failure after humidity exposure is often unsatisfactory. Given that such wood failure is an important requirement included in inter alia the Canadian Standard Association Norm CSA 112.9 for structural wood adhesives intended for exterior exposure, recent research has focused on improving the wet adhesion strength of polyurethane adhesives.
Custodio et al. in “A Review of Adhesion Promotion Techniques for Solid Timber Substrates”, The Journal of Adhesion 84, 2008, pages 502-529 proposes a number of adhesion improvement techniques for wood elements, including for instance corona discharge and flame treatments. However for practical, safety and environmental reasons, a number of authors have focused on the use of primers which are used to treat the surface of the wood prior to the application of the adhesive composition thereto.
International Patent Application Publication No. WO03/093385 (Huntsman International LLC) discloses the optional use of a surface treatment solution for the adhesion improvement of 1-component (1K) polyurethane adhesives on Southern Yellow Pine and Douglas Fir. These surface treatments may comprise aqueous solutions comprising from 0.05 to 10% by weight of either urea, polyvinylalcohol, salts of dodecylbenzene sulfonic acid or copolymers of ethylene with vinyl acetate. Whilst this document finds that urea solutions gave good results on Southern Yellow Pine—meeting the norms set by the ASTM D2559 on wood delamination—these solutions were not effective on Douglas Fir. Furthermore, this document teaches that the wood to which the surface treatment is applied must first be planed and sanded; without the sanding step, the aqueous solutions did not promote the polyurethane adhesive strength to the extent necessary to pass the wet exposure requirements of ASTM D2559. And it is noted that sanding is not usually carried out at plant scale nor is it allowed by official norms for structural wood products.
U.S. Pat. No. 5,543,487 (Vick et al.) discloses a hydroxymethylated resorcinol coupling agent including a formaldehyde-based, hydroxymethylated resorcinol as an active ingredient, which enables commercial thermosetting adhesives, such as bisphenol-A epoxy, phenol-resorcinol-formaldehyde, emulsion-polymer isocyanate, phenol-formaldehyde and resorcinol-formaldehyde, to develop bonds to a variety of untreated and preservatively treated woods. To form the coupling agent, formaldehyde and resorcinol are reacted under mildly alkaline conditions in situ for between 4 and 6 hours at a temperature between 65° and 85° F. before application to wood surfaces at the very high spread rate of 150 g/m2. In toto the bonding process of this document takes of the order of days and is too slow for economical application in a production plant. Moreover, the reactant formaldehyde is a known human carcinogen and its use necessitates precautionary and expensive plant construction.
International Patent Application Publication No. WO2007/095670 (Commonwealth Scientific and Industrial Research Organisation) relates to a waterborne primer composition comprising polyamine—preferably a polyethyleneimine of molecular weight in the range of from 500 to 3 million—and having a pH of at least 11.5. The document also relates to a method of improving the bonding of wood with coatings such as electrostatic coatings and to adhesives using this primer composition. In the enabled embodiments of this document, the wood specimens required sanding prior to the application of the primer: this is problematic for the reasons noted above and perhaps more so here as it is a two-step sanding process consisting of coarse sanding using first a coarse 80-grit sanding paper followed by fine sanding with a 150-grit sanding paper. Furthermore, whilst the dry tensile strength of the primer-containing samples improved clearly, the citation fails to report any results regarding wet adhesion strength, delamination or heat resistance as required by structural use standards.
There thus remains a need to develop a primer composition which can be used to enhance the adhesion strength, in particular the wet adhesion strength of a polyisocyanate adhesive composition within an adhesive system which does not suffer the aforementioned disadvantages of the prior art.